1. Field of the Invention
The present invention relates to a rotatable cylindrical magnetron sputtering apparatus having a shunt for fine-tuning the magnetic field.
2. Description of Related Art
A typical magnetron sputtering device includes a vacuum chamber having an electrode contained therein, wherein the electrode includes a cathode portion, an anode portion and a target. The term electrode is oftentimes referred to in the industry as a cathode. In operation, a vacuum is drawn in the vacuum chamber followed by the introduction of a process gas into the chamber. Electrical power supplied to the electrode produces an electronic discharge which ionizes the process gas and produces charged gaseous ions from the atoms of the process gas. The ions are accelerated and retained within a magnetic field formed over the target, and are propelled toward the surface of the target which is composed of the material sought to be deposited on a substrate. Upon striking the target, the ions dislodge target atoms from the target which are then deposited upon the substrate. By varying the composition of the target and/or the process gas, a wide variety of substances can be deposited on various substrates. The result is the formation of an ultra-pure thin film deposition of target material on the substrate.
Over the last decade, the cylindrical magnetron has emerged as the leading technology for sputtering coating on glass substrates. The rotating cylindrical target surface provides for a constant sputtering surface, thus eliminating the traditional erosion groove and large non-sputtered areas associated with planar targets. Further, the cylindrical target eliminates large areas of dielectric buildup that can lead to arcing, material flaking, debris, and other process instabilities. Although the rotatable cylindrical magnetron has its advantages over planar magnetrons, the shape of the magnetic field which determines everything from field uniformity and deposition rate to target utilization may still be optimized further to improve the performance of the sputtering application. The use of stationary profiled magnets can be used to control the shape of the magnetic field which optimizes the performance of the sputtering application. U.S. Pat. Nos. 5,736,019 and 6,171,461, which are incorporated herein by reference, disclose and attempt to overcome under utilization of target material via the use of stationary profile magnets. The above-identified patents are directed to magnetron sputtering electrodes that include a plurality of profile magnets, each magnet including a top portion with an apex, wherein each apex is positioned adjacent a target supporting surface in the cathode body. The magnet cooperates to generate magnetic flux lines which form enclosed-looped magnetic tunnels adjacent to the front sputtering surfaces of the target. As described in the above-identified patents, these profile magnets result in optimum utilization of target materials at a reasonable rate of utilization. A problem with the conventional planar magnet arrangement is that the magnets have flat upper surfaces and, therefore, the target which the material is to be sputtered from is not completely utilized.
The development of mid-frequency AC power supplies has enabled continuous long-term sputtering of targets which are utilized in a reactive gas to form dielectric or poorly conductive thin films. Albeit a dramatic improvement above planar targets used in planar cathodes, rotatable cylindrical targets still have a region just beyond the magnetron ends (i.e., turnarounds) which are not sputtered, but rather collect a portion of the sputtered thin film. When the sputtered material builds up in these turnarounds or unetched regions to a substantial thickness thus forming an insulating layer, this layer can become a source of arcing. Although enabling power supply technology has increased process stability of the deposition process, it has simultaneously introduced increased complexity and cost into the design and arrangement of the hardware associated with the cathode drive assembly which delivers this power to the target surface. The two most common problems associated with the delivery of high power, mid-frequency (20 kHz-120 kHz) current to the cathode are (1) the ability of the brush assemblies to carry sufficient current without overheating and eroding due to the “skin-effect” of these frequencies and (2) the inherent eddy current effects induced by these frequencies which can cause extreme localized heating of various components, particularly the support bearing. To circumvent the high current requirements, many manufacturers are using custom brush assemblies with high silver content in order to overcome the above-mentioned problems. The design and manufacture of custom brushes used in these assemblies are not only costly, but the material is very brittle which can lead to a short operating life. For example, one such solution for addressing the eddy current problem is to use a custom designed ceramic bearing, which is costly and difficult to replace quickly.
Therefore, it is an object of the present invention to improve the performance of the cylindrical magnetron sputtering application by using profiled magnet arrangements to increase the target utilization and the deposition rate while reducing the amount of target material on the chamber walls and shielding of a cylindrical magnetron electrode. It is a further object of the present invention to provide an improved drive assembly for a cylindrical magnetron electrode that is designed to reduce the cost and complexity of delivering high power AC current into a rotating shaft by using common and readily available components.
Further, it is known in cylindrical magnetron sputtering that the film thickness on the substrate can vary, even when the magnets and target are perfectly aligned. These coating thickness variations may be attributed to many influences on the sputtering process not associated with the magnets. Such influences may include vacuum system irregularities such as non-uniform anode distribution and non-uniform system pumping or gas flow across the length of the target. When irregularities in the thickness of the coating on the substrate occur, one can take steps to promote coating uniformity, such as altering the system pumping or gas flow, or raising and lowering the relative heights of the magnets. Such methods of fine tuning the sputtering have some limitations, however. For example, raising and lowering the magnets only allows for tuning on the scale of the fixed lengths of the magnets, and may not permit for finer tuning on a smaller scale. Accordingly, it is another object of the present invention to improve the means by which the magnetic field may be fine tuned to promote more uniform coating of the substrate.